Earth and Planetary Science Letters 240 (2005) 486 – 494
www.elsevier.com/locate/epsl
Biomass effects on stalagmite growth and isotope ratios:
A 20th century analogue from Wiltshire, England
J.U.L. Baldini a,*, F. McDermott a, A. Baker b, L.M. Baldini a,
D.P. Mattey c, L. Bruce Railsback d
a
b
UCD School of Geological Sciences, University College Dublin, Belfield, D4, Ireland
School of Geography, Earth & Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
c
Geology Department, Royal Holloway College, University of London, Egham, Surrey, TW20 OEX, UK
d
Department of Geology, University of Georgia, Athens, GA 30602, USA
Received 25 January 2005; received in revised form 16 June 2005; accepted 15 September 2005
Available online 24 October 2005
Editor: E. Boyle
Abstract
Increases in calcite deposition rates combined with decreases in y13C and y18O in three modern stalagmites from Brown’s Folly
Mine, Wiltshire, England, are correlative with a well-documented re-vegetation above the mine. Increased soil P CO2 resulted in
greater amounts of dissolved CaCO3 in the drip waters, which consequently increased annual calcite deposition rates. The absence
of deposition prior to 1916 (28 years after the mine was closed) indicates that vegetation had not yet sufficiently developed to allow
higher P CO2 values to form in the soil. Lower y13C values through time may reflect the increased input of isotopically light biogenic
carbon to the total dissolved inorganic carbon (DIC). y18O decreased synchronously with y13C, reflecting the increased importance
of isotopically light winter recharge due to greater biomass-induced summer evapotranspiration. This is the first empirical
demonstration that vegetation density can control stalagmite growth rates, y13C, and y18O, contributing critical insights into the
interpretation of these climate proxies in ancient stalagmites.
D 2005 Elsevier B.V. All rights reserved.
Keywords: stalagmite; carbon isotopes; oxygen isotopes; fractionation; vegetation; growth rate
1. Introduction
Stalagmites are widely regarded as critically important archives of terrestrial paleoclimates, but it is increasingly apparent that the mechanisms responsible for their
petrographic and stable isotope signals are inadequately
understood. Many stalagmites preserve visible petrographic [1,2], luminescent [3,4], and/or geochemical
* Corresponding author. Tel.: +353 1 7162148; fax: +353 1 283
7733.
E-mail address: [email protected] (J.U.L. Baldini).
0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2005.09.022
laminae [5,6] that not only enable the construction of
highly precise chronologies, but also provide valuable
paleoclimatic information. Variations in the thickness of
stalagmite laminae are interpretable in terms of key
climatic variables such as effective precipitation (principal control on drip rate) [7], temperature and soil P CO2
[8], and when combined with precise U/Th dates and
stable isotopic measurements, serve to quantify the timing and the magnitude of paleoclimatic events.
Previous studies in regions characterized by sharp
ecotone transitions have suggested that stalagmite y13C
fluctuations reflect climate induced shifts in vegetation
J.U.L. Baldini et al. / Earth and Planetary Science Letters 240 (2005) 486–494
type, primarily because plants utilizing the C3 photosynthetic pathway have significantly lower y13C values
than C4 plants [9–11]. However, recent research on a
stalagmite deposited during the last glaciation (MIS 3
and 4) indicates that warmer temperatures associated
with Dansgaard-Oeschger events promoted the development of local vegetation resulting in more biogenic
CO2 dissolved in seepage water, obviating the need to
appeal to changes in vegetation C3 / C4 ratio [12]. Low
y13C values (characteristic of biogenic carbon) coinciding with periods of rapid stalagmite growth were interpreted as representing increased surface vegetation
density [12]. Conversely, elevated y13C values were
interpreted as a representing a return to near full glacial
conditions, and these increasing y13C trends were often
truncated by a cessation of stalagmite growth indicative
of conditions unfavorable to vegetation [12]. A need
clearly exists for an improved understanding of the
processes responsible for temporal changes in stalagmite y13C, particularly in temperate regions where vegetation type is predominantly C3.
In this study we analyze laminae thicknesses, y13C,
and y18O of three modern stalagmites obtained from a
site that has undergone well-documented changes in
surface vegetation (Brown’s Folly Mine, SW England)
to establish a clear link between the extent of surface
vegetation and the stalagmite record. The changes observed in the temporal distribution of calcite deposition,
y18O, and y13C within the Brown’s Folly Mine stalagmites are similar in magnitude to variations recorded
during glacial–interglacial transitions for many stalagmites [13–16], suggesting that the density of vegetation
is a significant control on speleothem climate records.
2. Research setting
Brown’s Folly Mine, Bathford Hill, SW England, is
developed in the Bath Oolite member of the Jurassic
Great Oolite Series limestone at between 5–15 m depth
below the surface [17]. The mine was opened in 1836
for the extraction of building stone, thus constraining
the maximum age of its stalagmites. The mine was
worked until 1886, when all of its entrances were
sealed. Mining continued in other mines within Bathford Hill until 1904, when the area was converted into a
nature preserve and vegetation was allowed to re-establish itself. Aerial photographs of Bathford Hill indicate
that large portions of the hill were still barren in 1945
(Fig. 1a) but were gradually reclaimed by the secondary
deciduous forest (Fig. 1b,c). A photograph taken in
1907 (Fig. 1d) demonstrates that immediately after
the cessation of mining no substantial vegetation
487
existed on the area directly above the study site, contrasting sharply with the well-developed mixed ash,
sycamore, and oak forest currently in existence
(Fig. 1e). Native species of trees and shrubs were
preferentially allowed to return. A thin brown rendzina
soil overlies the mine [8]. The entrances to Brown’s
Folly Mine remained closed until cavers re-opened the
entrances in the 1970s [18]. The mine is in an area of
maritime temperate climate with a mean annual temperature of 10.0 8C and a mean annual precipitation of
842 mm. Precipitation is slightly more common during
the winter, but rainy weather occurs throughout the year
[19]. The site was chosen for this study because it
affords an excellent opportunity to test the links between surface vegetation, stalagmite morphology, and
stalagmite isotope geochemistry.
The three sampled stalagmites were obtained from
the same chamber, within ten meters of each other, but in
different years (BFM9: 1998, Boss: 1996, F2: 1998).
The chamber is located approximately 300 m from the
nearest entrance and is approximately ten meters wide,
ten meters long, and two meters in height. The temperature in the sampling chamber was measured as 10 F 0.4
8C, with no seasonal variation, and the humidity within
the sampling chamber is close to 100% [18], effectively
eliminating the possibility of evaporation. The P CO2 of
the sample chamber was measured as 0.0035–0.0040
atm [18]. All the entrances are at approximately the
same elevation, thereby minimizing airflow through
the mine. A similar thickness of bedrock exists directly
over the stalagmite sampling sites, though because the
overlying topography is an extremely heterogeneous
karst, the hydrological routing between the surface and
each of the three stalagmites could vary dramatically.
The amount of vegetation directly above all three stalagmites is similar, but localized heterogeneity may still
exist (e.g., more vegetation at the base of a doline).
3. Methods
Thin sections of the three stalagmites collected from
the floor of the mine (BFM9, Boss, and F2) were
assessed for variability in laminae structure, crystal
habits, and calcite fabrics with a petrographic microscope. Growth lamina thicknesses for BFM9, Boss, and
F2 were measured using high-resolution digital scans
and the drafting program CanvasR (error = +/ 0.001
mm), which allowed for additional magnification where
necessary and ensured that only continuous layers were
measured. Three independent laminae thickness counts
were conducted per stalagmite to ensure reproducibility.
Couplet thicknesses are reported as percents of the total
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Fig. 1. A) Oldest available aerial photograph of Bathford Hill demonstrating that large portions of the hill were devoid of vegetation in 1945. Large
arrow indicates location of Brown’s Folly tower, which is directly above sampling area. Orange shading indicates areas with minimal vegetation and
was determined by visual examination. B–C) Aerial photographs taken in 1968 and 1989, respectively, showing developing secondary forest. D)
Photograph of Brown’s Folly tower taken in 1907 demonstrating that the zone directly above the stalagmite sampling area was barren. E)
Photograph of Brown’s Folly tower taken in 1998 showing well-developed secondary vegetation now characteristic of the site.
calcite deposition (thickness of the stalagmite at the
location of the laminae count) to facilitate comparison
between the three stalagmites. Calcite powders (~1 mg)
of were obtained using a drill equipped with a 600 Am
drill bit at a mean spatial resolution of 1.2 mm and
stable isotope ratios (y18O and y13C) were analyzed
using a VG Prism Series 2 mass spectrometer at
Royal Holloway, University of London. The analytical
precision was 0.1x (1j) for both y18O and y13C. The
drill bit diameter results in temporal averaging (based
on the growth rates presented below) of approximately
4.5 yr for the first ten years of calcite deposited and 1.5
yr for the last ten years of calcite deposited for all three
stalagmites.
4. Results and discussion
All three stalagmites share the same basic petrography with varying degrees of internal layering consisting
of alternating bands of laterally continuous clear calcite
and dark inclusion-rich calcite (Fig. 2). The laminae
within the stalagmites were most likely deposited as
yearly couplets, similar to those found in previous
studies [20–22]. The dark layers of the couplets probably represent the annual autumnal/winter flushing of
organic compounds and debris related to leaf fall [23].
The calcite grew in large, columnar crystals oriented
parallel to the growth axis apparently unaffected by the
distribution of inclusion-rich and clear layers. No
growth hiatuses were observed in any of the stalagmites. Clear, euhedral crystal terminations were present
at the tops of BFM9, Boss, and F2, suggesting that the
stalagmites were still actively growing when collected.
Three radiocarbon measurements (AMS, Center for
Isotope Research, University of Groningen) successfully located the radiocarbon dbomb-spikeT occurring in
1964 within BFM9, providing additional chronological
control independent of that provided by the laminae
J.U.L. Baldini et al. / Earth and Planetary Science Letters 240 (2005) 486–494
489
Fig. 2. Photographs of stalagmites from Brown’s Folly Mine displaying characteristic layering. The windows to the right are a 5 magnification of
the layering compared to the panels to the left. The dashed line separates the stalagmite from the oolitic grainstone substrate.
counts [24]. Because the couplets are deposited annually, construction of a chronology is possible.
The total thickness of stalagmite BFM9 at the central
growth axis is 12 mm and it consists of approximately
67 annual couplets (67+/ 3, based on three replicate
counts). According to the laminae count with the greatest number of laminae, the stalagmite nucleated no
earlier than 1928 (Fig. 3). Couplet thicknesses decrease
until 1960 when annual calcite deposition gradually
begins to increase. A dramatic increase in couplet
thickness occurs in 1980, and the annual amount of
calcite deposited from 1980 is approximately 4 times
the annual deposition rate for the period between 1940
and 1975. The total thickness of stalagmite Boss at the
central growth axis is 20 mm and it consists of 77 +/ 3
annual couplets, suggesting that the stalagmite nucleated no earlier than 1916 (Fig. 3). Annual calcite
deposition rates increase gradually but remain low
until 1975, when couplet thicknesses increase dramatically. The thickness of stalagmite F2 at the central
growth axis is 18 mm and it consists of approximately
62 +/ 8 annual couplets, suggesting that F2 nucleated
no earlier than 1931 (Fig. 3). Very little deposition
occurs during the first 28 years of growth. Deposition
rates double around 1960, and increase even more
dramatically in 1991.
The temporal trends in calcite deposition rates are
broadly similar in all three stalagmites. Differences in
the second-order variability between the records probably result from heterogeneities in the hydrological
routing of the drips feeding the stalagmites. Although
the mine entrances were closed in 1886, no deposition
occurred in any of the three stalagmites prior to 1916.
The re-opening of the entrances to the mine in the
1970s theoretically could have increased the growth
rates of the stalagmites, but because the growth rate
increases occurred at different times, the sampling site
was not located close to an entrance, and because the
observed isotopic shifts (discussed later) are not consistent with increased ventilation, the re-opening of the
mine entrances is not believed to have affected the
growth rate of the stalagmites. Photographic evidence
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Fig. 3. Couplet thickness records for BFM9, Boss, and F2. Each individual record averages couplet thicknesses for each year obtained in three
independent layer counts. Error bars indicate error in the chronology and are based on the difference between the total numbers of layers counted in
the three independent counts per stalagmite. Chronological uncertainty is therefore smaller near the tops of the stalagmites and increases toward the
bases. Shaded bars represent areas of where calcite petrography complicated layer counting, generally due to less well developed dark laminae. Also
marked are the dates when the mine was closed (1886) and when the area was turned into a nature preserve (1904). Lowermost panel shows annual
rainfall (Long Ashton Meteorological Station, 40 km from site) and temperature (Central England Temperature Record, [30]). Three samples were
drilled from BFM9 for radiocarbon analysis. Two samples corresponding to calcite deposited during the years 1944–1958 and 1985–1991 were
enriched in 14C, indicating that deposition occurred during or after the period of abundant nuclear weapons testing (1945–1964). The remaining
sample (corresponding to calcite deposited during the years 1929–1931) was not enriched in 14C, indicating that deposition occurred before the
initiation of nuclear weapons testing. These points were selected to verify the chronology derived from the counting of couplet thicknesses, but were
not directly applied towards the development of the chronology. Absolute 14C concentrations were unimportant; only their enrichment relative to the
dbomb spikeT was interpreted.
(Fig. 1) indicates that no substantial surface vegetation
existed prior to the conversion of the area to a nature
preserve in 1904, strongly suggesting that the absence
of vegetation resulted in a groundwater with low levels
of dissolved CO2, minimizing bedrock dissolution and
subsequent calcite deposition in the mine. Thus, increasing annual calcite deposition rates through time
may reflect the gradual re-establishment of the forest
above the mine, unaccompanied by any systematic
long-term change in either temperature or precipitation
(Fig. 3). The difference in the timing of the onset of
rapid calcite deposition at the three sites probably
results from differences in the local hydrological routing and local heterogeneities in the distribution of
surface vegetation. The strong similarity between the
temporal calcite depositional patterns observed in the
modern stalagmites discussed here and those characteristic of glacial–interglacial transitions [13,25,26] demonstrates that calcite deposition is dependent on
surface biomass and respiration. Previous studies focusing on stalagmite deposition through time in Europe
demonstrated that stalagmite growth rates decrease during glaciations [13,25,26]. Because the anthropogenic
changes in surface vegetation that occurred above
Brown’s Folly Mine approximate those in mid and
high latitude regions caused by natural climatic shifts,
we suggest that changes in temperature and meteoric
precipitation accompanying climatic transitions affect
J.U.L. Baldini et al. / Earth and Planetary Science Letters 240 (2005) 486–494
the rate of calcite deposition primarily by inducing
changes in the biomass/bioproductivity overlying the
cave.
All three stalagmites have similar y13C values
(means: BFM9 = 9.3 F 0.6x, Boss = 9.6 F 0.5x,
F2 = 8.8 F 0.5x, PDB) that decrease through time
(Fig. 4), broadly correlating with increasing laminae
thickness. This is consistent with the signal expected
from the re-establishment of vegetation (i.e., reduced
y13C values in response to a greater contribution of
isotopically light biogenic carbon). The absolute y13C
values are similar to those found in ancient stalagmites
in other studies, but the change in values
(BFM9 = 1.86x, Boss = 1.23x, F2 = 2.12x)
marking the transition from low-biomass to high-biomass conditions is slightly less than that found in
ancient stalagmites. For example, Genty et al. [12]
found that y13C values decreased by approximately
4x over the two thousand years immediately following
a cold event (coincident with Heinrich event 6) which
was manifested in the stalagmite record as a growth
hiatus. This difference in magnitude of the y13C shift is
probably attributable to the lack of sufficient soil development overlying Brown’s Folly Mine. Whereas in
a natural situation soil development would parallel
transitions through successive biomes controlled by
491
gradual climate change, vegetation growth over
Brown’s Folly Mine was not constrained by any climatic parameters resulting in soil development that lags
vegetation development. Additionally, calcite deposition would not have occurred until the soil was sufficiently developed to produce a soil gas P CO2 greater
than the P CO2 of the poorly ventilated mine atmosphere. Therefore, the earliest calcite deposition in the
mine may have low y13C values because deposition
only occurred after some soil development, consequently reducing the range of observed y13C values
in the three stalagmites. The presence of some soil
organic carbon developed before the mining related
disturbance to the site may have also reduced y13C
values. The y13C data presented here demonstrate that
stalagmites are able to successfully record vegetation
shifts, and suggest that the y13C variations observed in
previous studies may partly result from changing
amounts of biomass as well as shifts in the distribution
of C3/C4 plant species.
Oxygen isotopic variations decrease through time
and correlate well with y13C for all three stalagmites.
Increases in cave air temperature can decrease y18O
values of a stalagmite ( 0.24x per 8C at 10 8C)
because of changes in the fractionation factor between
water and calcite [27,28]. The temperature change cal-
Fig. 4. Carbon and oxygen isotope changes through time for stalagmites BFM9, Boss, and F2 (y13C = solid line; y18O = dashed line). Overall
decreases in both y13C and y18O are apparent. Modeled y18O values of calcite precipitated through time based solely on changes in the seasonality
of rainfall do not reproduce the decreasing trend in values apparent for the three stalagmites.
492
J.U.L. Baldini et al. / Earth and Planetary Science Letters 240 (2005) 486–494
culated by using the difference between the mean y18O
values for the first 20 years of deposition for each
stalagmite and the last 20 years (BFM9 = 1.5 8C,
Boss = 1.2 8C, F2 = 3.0 8C) significantly overestimates
the actual warming (0.34 8C, Central England Temperature record [29,30]) for the same periods. Besides
being affected by temperature variations, stalagmite
y18O values are also influenced by fluctuations in rainfall y18O. This explanation is also improbable because
available data (1982–2001, GNIP, Wallingford, UK)
suggest that systematic long-term changes in mean
annual rainfall y18O have not occurred recently. A
possible explanation for the covariance observed in
y13C and y18O is that it is controlled by kinetic fractionation. The entrances to the mine were re-opened in
the 1970s [18], potentially resulting in ventilation leading to a sudden reduction in the P CO2 in the mine and
disequilibrium fractionation (e.g., [31]). A test for isotopic equilibrium conducted on a layer within F2 deposited circa 1984 displays a positive correlation
between y13C and y18O (r 2 = 0.74), indicating the possibility of kinetic fractionation [32–34]. Oxygen isotope
ratios decrease 0.56x towards the flanks of the stalagmite and y13C values decrease by 1x. However, kinetic
fractionation would result in both y13C and y18O enrichment [31,35], which is the opposite of what is
observed in the three stalagmites during the 20th century and precludes kinetic fractionation as a possible
explanation for these long-term trends. Because the
progressive isotopic depletion observed in all three
stalagmites cannot result from kinetic fractionation,
additional Hendy tests were not conducted. Additionally, any sudden changes in ventilation should affect
isotopic values and growth rates for the three stalagmites simultaneously, which is not observed.
Another possible control on the y18O of the stalagmites involves changes in the seasonal distribution
of rainfall. Existing monthly rainfall data demonstrates that seasonal rainfall patterns changed abruptly
in 1974: summer rainfall decreased and winter precipitation increased [36,37]. This may have skewed
groundwater recharge towards isotopically light winter precipitation [mean y18OJULY = 4.46, mean
y18OJANUARY = 8.01 (1982–2001, GNIP, Wallingford,
UK)], resulting in reduced y18O values in the stalagmites. However, stalagmite y18O values resulting exclusively from variations in rainfall seasonality were
modeled and demonstrate no decreasing trend for the
period 1921–1991. Therefore, if the only variables
affecting the y18O of the stalagmites were the y18O of
the rainfall combined with changes in rainfall seasonality, no decreasing trend in stalagmite y18O would
exist. Additionally, the observed decreases in y18O
occur gradually, do not occur simultaneously in the
three stalagmites, and are correlated to y13C changes.
For these reasons, the slight shift in the seasonal distribution of precipitation cannot result in the observed
y18O trend in the stalagmites.
The data therefore suggest that the decrease observed
in oxygen isotopes reflects the gradual development of
vegetation on the surface through the 20th century. A
soil moisture deficit often exists at the study site during
the summer [mean water excessJULY = 4.5 mm, mean
water excessJANUARY = 64.8 mm (1921–1991, Bath Meteorological Station)], but this was probably lower before the establishment of vegetation. It is proposed here
that increased evapotranspiration during the summer
months associated with increased vegetation density
may skew groundwater recharge towards isotopically
light winter precipitation, resulting in reduced y18O
values in the stalagmites. This would also explain the
observed correlation between y18O and y13C throughout
the record; increased biological activity would increase
the importance of winter precipitation (lowering y18O)
while simultaneously increasing soil P CO2 (lowering
y13C).
5. Conclusions
The growth rate and y13C variations of the three
modern stalagmites observed in this study test the
ability of stalagmites to record changes in the density
of surface vegetation, analogous to those that occurred
during glacial/interglacial transitions. Annual calcite
deposition rates increase through time, reflecting an
increasing difference between soil and mine atmosphere P CO2 due to the re-establishment of vegetation
on a previously barren mining site. y13C values decrease through time due to increased input of isotopically light biogenic carbon to the total dissolved
inorganic carbon (DIC). Increased summer evapotranspiration may increase the importance of isotopically
light winter recharge to the aquifer relative to summer
precipitation, resulting in decreases in y18O. This illustrates another potentially critical mechanism capable of
altering y18O in stalagmites and needs to be researched
further. The research presented here strongly corroborates previous studies suggesting that periods of reduced calcite deposition and increased y13C values
are associated with colder/drier climatic perturbations.
Our conclusions demonstrate that stalagmite growth
rates, y13C, and y18O are at least partially controlled
by vegetation, which in glacial periods is dependent on
climatic conditions.
J.U.L. Baldini et al. / Earth and Planetary Science Letters 240 (2005) 486–494
Acknowledgements
We thank the Avon Wildlife Trust for allowing
access to the study site. We also thank Ian Fairchild
and Tim Atkinson for helpful discussions. This manuscript benefited from reviews by Pete Smart and two
anonymous reviewers. This research was supported by
Enterprise Ireland Basic Research grant SC/99/012.
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